The invention relates to a digital printing or copying machine for one-sided or double-sided printing of a substrate using a process wherein at least two melt areas on the substrate are arranged in succession and laterally offset to one another.
Digital printing or copying machines work, for example, using the electrographic process, in which a latent electrostatic image is developed by charged toner particles. The latter are transferred to an image receiving substrate, hereinafter the substrate for short. Then the developed image, which has been transferred to the substrate, is fixed by the toner particles being heated and melted thereon. To melt on the toner particles, contact making methods are often used, in which the toner particles are brought into contact with the heater, for example hot rollers or drums. The disadvantage here is that generally it is necessary to use silicone oil as the separating agent which is designed to prevent adhesion of the melted toner to the heater. Furthermore, building and maintaining these contact making heaters are complex and thus operating costs are high. In addition, the fault rate caused by the contact making heaters is relatively high. To fix the toner which has been transferred for example to paper, furthermore noncontact heaters and processes are known, in which for example the toner particles are melted using thermal/microwave radiation or hot air.
In the contact making and noncontact melt on processes, for example toners are used which have a glass transition temperature (TG) which is in the range from 45xc2x0 C. to 75xc2x0 C. The glass transition temperature at which the toner, proceeding from the solid state, begins to become soft, can be influenced by the choice of raw materials and by adding certain additives to the toner. In a fixing device which has at least one heater for the toner, both the toner and also the substrate itself are heated. To be able to ensure good fixing of the toner on the substrate, the surface temperature of the substrate must be in the area of the glass transition temperature of the toner or above. The toner reaches or exceeds the glass transition temperature (TG) in the area of the heaters.
Printing and copying machines are known in which the substrate is printed on both sides, for printing of the front and back the same image generating and transfer device and heater or one separate image generating and transmission device and heater at a time being used. To fix the toner image, the substrate is often guided past at least one image generating and transfer device and the assigned heater using a transport belt on which the substrate lies. First of all, a first toner image is transferred to the first substrate side and fixed. Then a second toner image is transferred to the second substrate side and fixed. When the second toner image is melted, therefore the first substrate side with the already fixed first toner image located on it adjoins the transport belt. The disadvantage here is that while the second toner image is being melted, the first toner image can be heated to such an extent that it becomes soft and tends to stick to the transport belt. This can lead to several undesirable effects: The sticking can lead to problems in the transfer of the substrate from the transport belt to a following part of the machine until the substrate piles up. Furthermore, the appearance of the toner image can change in the areas in which it has adhered to the transport belt. This leads to problems in image quality, for example, the toner image has a nonuniform gloss.
The object of the invention is to devise a machine and a process in which double-sided printing of a substrate with simultaneously high quality of the toner images applied to the front and back of the substrate is possible.
To achieve this object, a digital printing or copying machine is proposed which has at least one fixing device which is used for fixing of a toner image which has been transferred to the substrate. The toner image can be monochrome or polychrome. In conjunction with this invention, a xe2x80x9ctoner imagexe2x80x9d is also defined as a coating which has at least one toner image. The substrate can for example be a sheet or continuous web which consists of paper or cardboard. To fix the liquid or dry toner on the substrate, it is routed past at least one heater which is part of the fixing device. To move the substrate in the area of the heater along the transport path there is a transport device which has several transport elements. The printing or copying machine of the invention is characterized in that the heater has at least two melting areas on the substrate, which viewed in the substrate transport direction are located in succession and laterally offset to one another. The toner image is therefore not completely melted and fixed in one process, but at least two component melting processes take place in which only one strip of the toner image which has a certain width is melted at a time. This makes it possible to arrange the transport elements distributed around the melt areas of the heater. This means that there are no transport elements or other guide elements which come into contact with the top and bottom of the substrate in the melt areas of the heater. If therefore when the first toner image which has been transferred to the top of the substrate is melted there should be a first toner image which has been fixed in a preceding treatment process on the bottom of the substrate and which can be heated to such an extent that it remelts, the second toner image cannot stick anywhere since, as stated, it has no contact with the surface. Until the first and/or second toner image comes into contact with the transport element or another surface, it is cooled to such an extent that there will be no adverse effect on the image quality or sticking of the substrate on this surface. Because the substrate has no contact with the surface in the area in which a part/section of the toner image is melted, a high quality of the toner images which have been applied to the front and back of the substrate can be ensured. Sticking of the toner images to a surface, for example to a transport element, is essentially precluded, since contact only occurs when the toner image is in the solid state again.
In conjunction with this invention the xe2x80x9cmelt areaxe2x80x9d of the heater is the area or the surface of the toner image which can be melted using the heater. The melt area, viewed in the substrate transport direction, has a width which is smaller than the total width of the toner image. Therefore simply one strip shaped toner image section is melted, not the entire toner image, by the heater. If the heater is formed for example by a radiation device which exposes the toner image to electromagnetic radiation, the melt area of the radiation device on the substrate is that part of the toner image which is melted by the radiation.
According to one development of the invention, it is provided that the melt areas overlap one another on the toner image to be fixed. Overlapping of the melt areas is preferably relatively little so that a toner image section/strip as small as possible is melted several times. The overlapping of the melt areas ensures that for a lateral offset of the substrate which has been displaced in the transport direction, in fact the toner image is also melted over the entire width and no unfixed toner image areas remain. Alternatively it is also possible for the melt areas not to overlap one another on the toner image to be fixed, so that the toner is not repeatedly melted.
The heater which the toner image is melted without contact, i.e., without mechanical contact, exposes the toner image to thermal and/or microwave radiation and/or hot air or steam. It is easily possible to implement several melt areas at a distance from one another on the toner image using only one heater. To do this the thermal/microwave radiation or the hot air must accordingly be applied specifically to the toner image. In another embodiment of the invention, the fixing device has at least two heaters which can preferably be controlled independently of one another and which each have at least one melt area on the substrate. Of course, it is also possible for one separate heater at a time to be used for each of the melt areas on the substrate.
To achieve this object, a process is furthermore proposed in which first of all a first toner image is transferred to the first side of the substrate and the substrate is routed past the first melt zone of a fixing device of the digital printing or copying machine in which the process is used. In the first melt zone a first toner image section is melted, i.e. a lengthwise strip of the toner image which viewed in the substrate transport direction has a width which is preferably smaller than the entire width of the toner image. By moving the substrate along its transport path it travels from the first melt zone into a following, second melt zone in which the second part of the toner image is melted. Therefore, several melting processes take place, only over one section/strip of the toner image at a time being heated to such an extent that the toner becomes liquid and can deliquesce. The number of melt zones is not limited to two and can easily be up to ten or more. It is important that the substrate in the areas in which melted toner is located has no contact with a surface, for example, a transport element. Since the toner image sections melted in the respective melt zone have a clearly smaller area compared to the entire toner image area, the substrate can be supported around the melt area which is active at the time, for example, by transport elements so that slack of the substrate in the melted toner image section essentially hardly occurs and therefore the melting process and reliable continued transport of the substrate are not influenced.